The IMS (IP Multimedia Subsystem) domain is part of mobile communications networks as standardized by the 3GPP (3rd Generation Partnership Project). Other domains are the CS (Circuit Switched) and PS (Packet Switched) domains. IMS based services may be provided using PS bearers or CS bearers for the media.
IMS Centralized Services (ICS) are defined in the 3GPP TS (Technical Specification) 23.292 for providing communication services based on IMS mechanisms while using CS access for the media bearer. For example, ICS provides mechanisms to support the use of a CS media bearer for an IMS (multimedia) session and defines signalling mechanisms between the UE (User Equipment) and IMS for supporting service centralization when using a CS access. ICS thus allows IMS user sessions based on CS bearers to be treated as standard IMS sessions for the purpose of service control and service continuity.
IMS Service Continuity is a service of IMS which supports the use of session transfer mechanisms to maintain service continuity in the event of terminal mobility and/or mobility between terminals in case such an event is not hidden from the IMS session layer (as, e.g., in case of mobility between CS accesses) and thus service continuity could not otherwise be maintained. Session transfer mechanisms support a transfer at the IMS-level of one or more of the session signalling paths and/or associated media paths of an ongoing IMS session while maintaining session continuity. Session Transfer incorporates at least one of access transfer and inter-UE transfer.
Both IMS Centralized Services and IMS Service Continuity are provided by a SIP (Session Initiation Protocol) application server in IMS termed SCC AS (Service Centralization and Continuity Application Server) as described in TS 23.292 and 23.237.
Session transfer procedures are initiated by the UE and are executed and controlled by the SCC AS. When the UE determines that access transfer is desirable and possible, a registration in IMS is performed by the UE via the transferring-in access network if possible and if the UE has not already registered itself via the transferring-in access network. A new access leg is established by the UE towards the SCC AS. Signalling and bearer resources are allocated in the transferring-in access network and the user's sessions are transferred from the transferring-out access network. The SCC AS executes access transfer procedures. Eventually resources in the transferring-out access network are released.
Additionally, SRVCC (Single Radio Voice Call Continuity) mechanisms for session transfer from an E-UTRAN (Evolved UMTS Radio Access Network) to CS are specified in the TS 23.216 for CS calls that are anchored in IMS.
With regard to session transfer, the SCC AS has to correlate an incoming session transfer request with an existing session. However, in IMS a user can have a (public) user identity shared among several sessions/devices such that the called party is presented with one and the same identity although using, e.g., different devices. Thus, the user may own multiple sessions based on the same identity. In case of several sessions from one and the same UE, it is common understanding in the competent 3GPP Service Architecture Workgroup to identify the last established active session with speech media as the session to be transferred. In case of several sessions sharing the same public user identity and related to different UEs, there is no common understanding on how to identify in the SCC AS the user device/session to be transferred.